1. Field of the Invention
The present invention relates to a plasma cutter which performs thermal cutting of an object to be cut such as a metallic plate or the like by emitting plasma ark, and to a plasma cutter power supply system for a plasma cutter.
2. Prior Art
A typical automatic plasma cutter comprises: a table upon which a piece of material to be cut, such as a steel plate or the like, is mounted; an XYZ drive system for driving a plasma torch with servo amps and servo motors or the like in X, Y, and Z directions; a plasma cutter power supply device for generating a plasma ark with a plasma torch; a gas supply device for supplying gas to the plasma torch to be ionized; a cooling water device for cooling a nozzle and an electrode included in the plasma torch; and an NC controller or the like for controlling the above described devices, according to a numerical control (NC) processing program, so as to emit a plasma ark from the plasma torch while moving the plasma torch relative to the material to be cut.
Furthermore, the plasma cutter power supply device typically comprises: an inverter; a main circuit which supplies electrical current for a plasma ark to the plasma torch, and having a DC constant current power supply circuit; a high frequency generation circuit for superimposing a high voltage for igniting a pilot arc between the electrode and the nozzle of the plasma torch upon the output voltage of the main circuit; a pilot circuit which applies the output voltage of the main circuit between the electrode and the nozzle during ignition of the pilot arc, and which thereafter performs changeover so as to apply this output voltage between the electrode and the material to be cut, so as to convert the pilot arc over to a main arc; and a power supply control device for controlling the main circuit, the high frequency circuit and the pilot circuit so as to ignite the pilot arc, and so as subsequently to keep the main arc sustained. It should be understood that the plasma cutter power supply device which comprises these elements is generally contained within a single chassis. While the electrical current of the main arc which is supplied from this type of plasma cutter power supply device varies according to the nature of the material to be cut and its thickness and the like, it may attain a high value such as several hundreds of amperes. Accordingly, it becomes necessary to provide a main circuit of high capacity.
In order to provide a power supply circuit of high capacity of this type, a technique is per se known of connecting a plurality of inverters or power modules of low capacity in parallel.
For example, there is per se known a power supply device for a welding machine which comprises a plurality of inverters of low capacity in parallel, with one among these inverters being taken as being a master while the others are slaves, and with all of these inverters being driven by a control signal from the master (for example, refer to Patent Document #1). Moreover, according to requirements, in such a power supply device, any one of the inverters can be set as the master. Furthermore, there is also per se known a power supply device for a plasma cutter in which, along with a control module, a plurality of power modules are stacked together so as to be removable (for example, refer to Patent Document #2).
With a power supply device as described in Patent Document #1, if a fault develops in one of the power units, then it will be sufficient to repair or to exchange this power unit. Moreover, it is possible to increase the electrical current capacity of such a power supply device by increasing the number of power units.
Patent Document #1: Japanese Laid-Open Patent Publication Heisei 8-1350.
Patent Document #2: U.S. Pat. No. 5,189,277.